1. Field of the Invention
The invention relates to the production of 7-ethyl indoles.
2. Prior Art
Indole has been produced from 2-ethylaniline with the help of dehydrogenation catalysts at an elevated temperature between 500.degree. and 700.degree. C.
According to German OS No. 2,049,752 and German OS No. 2,148,961, 2-ethylaniline is converted into indole in the presence of (i) water and oxygen or (ii) an oxygen-containing gas. It was later shown that a corresponding dehydrocyclization in a simplified process can be carried out without the use of oxygen or an oxygen-containing gas with better space-time-yields (Swiss Pat. No. 589,054).
7-Alkyl indoles are accessible as a result of the catalytic dehydrocyclization of the corresponding 2,6-dialkylanilines, but, contrary to indole itself, only with a low selectivity. Thus, 7-ethyl indole has only been obtained from 2,6-diethylaniline with a 40 percent selectivity (U.S. Pat. No. 3,441,569). Other production methods for 7-alkyl indoles are technically expensive or have a low selectivity. Thus, 7-ethyl indole can be obtained from 2-ethylaniline with the help of AlCl.sub.3 but with only a 20 percent yield [J. Med. Chem., (1980), 23, No. 11, p. 1224].
In the case of the catalytic dehydrocyclization of 2,6-diethylaniline in the presence of water (Swiss Pat. No. 589,054), a mixture of quite varied compounds is obtained. The 7-ethyl indole contained therein as the main component cannot be obtained easily by distallation since the by-products containing vinyl groups are inclined to spontaneous polymerization reactions. As a result of that fact, technical problems arise and there are high losses of substance and quality.
It is known from U.S. Pat. No. 3,894,042 that 7-ethyl indole can be produced by the catalytic conversion of 2,6-diethylaniline in the presence of steam with the exclusion of oxygen in a per-pass yield of a modest 13.2 mol percent.